The pop elimination system (i.e. pop elimination in an audio amplification system) can be a central element of an audio power conversion system.
Most audio power converters are based on a PWM modulator (digital modulator or analogue modulator) that converts a pulse-code-modulated signal (PCM) received from a source such as a CD-player, or an analogue signal preceded by a D/A converter, to for instance pulse-width-modulated signals (digital or analogue PWM modulator). Alternatively the modulator is analogue and is preceded by a D/A converter.
The output signal of the modulator is fed to a power stage where it is amplified. A typical power converter includes a switching power conversion stage, a filter and an analogue control system.
At start-up and close-down of the audio power conversion system a general problem is the presence of an audible signal at the output of the system even though there is no input signal applied to the audio power conversion system. The signal at the output at start-up and close-down is called pop.
A contributing source to the pop can be a DC step because the control system adds DC or because the PWM modulator adds DC.
Another contributing source of the start-up pop can be transients when the control system is started up from a saturated position.
A contributing source of the close-down pop can be non-controllable conduction of the MOSFET at close-down. Depending on the close-down timing in the PWM pulse, the current flowing in the filter can change the close-down timing of the MOSFET.
For eliminating the pop it is desirable to disconnect the output of the amplifier from the transducer at amplifier start-up and amplifier close-down. After the amplifier start-up it is desirable to connect the amplifier slowly to the transducer so that any DC output of the amplifier is slowly ramped at the transducer. Before amplifier close-down it is desirable to disconnect the amplifier output from the transducer so that any DC output of the amplifier is slowly ramped at the transducer. Therefore, it is desirable to include a switch after the power stage, as shown in FIG. 1.
In FIG. 1 there is shown an amplifier system including a switch in serial with the output of the amplifier. However, the switch shown in FIG. 1 increases the output impedance of the amplifier and furthermore the switch has to be able to handle any output current from the amplifier For this reason, high performance demands are made on the switch.